Power monitoring device for powerful fiber laser systems

ABSTRACT

A pig-tailed optical component used in a powerful fiber laser system is configured with a power monitor unit. The monitor unit has a plate-shaped beam splitter operative to reflect portions of at least one of respective forward and backreflected light signals, and multiple photo-detectors.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a powerful fiber laser system and, inparticular, to an optical coupler configured to monitor the power offorward and backreflected light signals propagating along a light pathin forward and backward directions, respectively.

2. Prior Art Discussion

A powerful fiber laser system is typically includes one or more lasercascades and capable of outputting tens and hundreds of watts. A lightsignal propagating along a powerful fiber laser system may vary within abroad range. The instability of the propagating signal detrimentallyaffects the task to be performed by a powerful laser system and thefunctionality of the system's components. To monitor the variation ofpower of light signals, optical laser systems are provided with taps.The purpose of such taps is to bleed off a small portion of opticalsignal to analyze the signal for desirable characteristics by aphoto-detector.

Quite often, to prevent detrimental effect of light backreflection thatmay be caused by inner obstacles, such as splices coupling adjacentfibers, optical isolators are coupled between the cascades. Thebackreflection can be also caused by an outer obstacle, such as thesurface to be processed during, for example, cutting and weldingprocesses. Typically, a hybrid structure configured with an isolator andtap is installed in a powerful laser system

The taps alone or in combination with isolators come in a variety ofconfigurations. FIGS. 1 and 2, for example, illustrates a multi-cascadedfiber laser system 10 including an input cascade Li 11 and at least oneoutput cascade Lo 12. A power monitor 14 preferably, but notnecessarily, is coupled to the output of output cascade 12 and includesserially coupled an isolator core and a fiber tap. The fiber tap isconfigured with a fiber tap source and a photo detector 16, asillustrated in FIG. 2. Typically, detector 16 is located adjacent to afiber bent or a taper where leaking light of the propagating signal maybe sensed by detector 16. Based on multiple measurements, the stabilityof such a tap, i.e., the ratio between the measured power and the actualpower of the propagating signal, is high and may reach about 10%. As aresult, the measurement data of the actual power may be imprecise andlead to unsatisfactory performance of the laser system.

A need, therefore, exits for a power monitor operative to provideimproved measurements of the power of light signals generated by apowerful laser system.

A further need exists for a photo detector configured to withstandrelatively high powers of the tapped signal.

SUMMARY OF THE INVENTION

These needs are satisfied by a power monitor unit configured inaccordance with the present disclosure. The disclosed powerful fiberlaser system includes, among others, an isolator core provided with atap component.

In accordance wit one aspect of the disclosure, the monitor includes asemi-transparent plate entrained by light which propagates from an inputfiber to an output fiber through an isolator core. The plate has twoopposite faces, at least one of which is covered by a reflectivecoating. The coated face of the plate allows to reroute or tap a smallportion of a forward propagating and backreflected lights, which isreflected from internal or external obstacles, to one or more photodetectors. Alternatively, the opposite faces can be covered byrespective reflective coatings. As a consequence, one of the coatedfaces taps a forward propagating light, whereas the opposite face taps aportion of the backreflected light; the tapped lights are sensed byrespective photo detectors. The configuration of the disclosed powermonitor allows for the increased stability of the measurements.

The light tapped off by the plate is still quite powerful to saturate,destroy or, at least, cause a photometer to malfunction. Accordingly, inaccordance with a further aspect of the disclosure, the face of thephotometer has a diffuser substantially weakening the received light. Asa result, the reliability of the disclosed system is improved.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the disclosure willbecome more readily apparent from the following specific descriptionaccompanied by the drawings, in which:

FIG. 1 is a diagrammatic view of the known powerful fiber laser systemprovided with a power monitor;

FIG. 2 is a power monitor or hybrid isolator/tap unit configured inaccordance with the prior art;

FIG. 3 is a power monitor unit used in a powerful fiber laser systemconfigured in accordance with the present disclosure;

FIG. 4 an enlarged view of the power monitor of FIG. 3; and

FIG. 5 is a view of photo detector configured according to the presentdisclosure.

FIGS. 6A-6C are respective diagrammatic views illustrating variouspractical application of the power monitor unit of FIGS. 2-5.

SPECIFIC DESCRIPTION

Reference will now be made in detail to the disclosed system. Whereverpossible, same or similar reference numerals are used in the drawingsand the description to refer to the same or like parts or steps. Thedrawings are in simplified form and are far from precise scale.

FIG. 3 illustrates a signal power monitor system including a pigtailedlinearly-polarized isolator 20 typically intended for use in a powerfullaser system LS, which is shown in a highly diagrammatic manner and mayinclude one or multiple cascades. The signal power monitor system ispreferably coupled to the output cascade, but may be located between thecascades. The isolator 20 includes an upstream fiber 22 carrying a lightsignal Ii along a light path to a downstream fiber 24. Optically coupledbetween upstream and downstream fibers 22 and 24, respectively, is anisolator core 26, which is provided with a tap coupler monitor 32 andflanked by input and output collimators 28 and 42, respectively.

In operation, input signal Ii is emitted from input fiber 22 and focusedby input collimator 28 so as to propagate in a forward direction Dfthrough isolator core 26. The isolator core 26 has a well knownstructure including an upstream polarizer 34, a 45° optically activerotator element 36, a Faraday rotator 38 and an output polarizer 40 alloptically connected to one another. The rotation of the plane ofpolarization provided by Faraday rotator 38 in one direction allowslight to pass through both polarizers 34 and 40, respectively, whichpolarize light in orthogonal planes, whereas, in the opposite direction,the plane of polarization is rotated so that the passage of the lightthrough isolator core 26 is blocked. As known, a polarizer is a devicefor producing light beam polarized in a specific direction. The inputpolarizer 34 is configured as a plate with a polarizing coating and istypically aligned to a linear polarization angle of input light Ii. Thepolarizing coating is important within the context of high power lasersystems since it is capable of withstanding high powers without beingdestroyed. The isolator 20 may have an additional input polarizer 34′ inorder to provide for polarizing ability. The output polarizer 40 isaligned to a non-parallel polarization angle so as to transmit thispolarization state at the angle of 90° or 0°, as known to one ofordinary skills sin the art.

Referring to FIG. 4 in addition to FIG. 3, tap coupler monitor 32includes a splitter 46 optically coupled between output polarizer 40 andoutput collimator 42 and photo detectors 44 and 52, respectively. In theforward direction, splitter 46 is operative to branch a small portion,tap signal Iti, of Ii signal off its light path through a short-focallens 53 to photodetector 44 capable of sensing tap signal Iti. Thesplitter 46 is configured as a rectangular plate having opposite faces48 and 50 which extend in a non-orthogonal plane with respect to anoptical axis A-A′ of system 20. Note that splitter 46 can be installedat any location along the optical path between input and outputcollimators 28 and 42, respectively. Accordingly, the location ofsplitter 46 as shown in FIG. 3 is just exemplary.

In accordance with one embodiment, both faces 48 and 50 (FIG. 4) ofsplitter 46 are covered by respective anti-reflective coating filmscapable of reflecting only a small portion of forward light signal Iiand backreflected light signal Iir to respective photo-detectors 44 and52, which are operative to simultaneously sense the forward propagatingand backreflected lights. The experimental data shows that the stabilityof tap coupler monitor 32, that is a ratio Pti/Pi between the power Ptiof tap signal Iti and the power Pi of light signal Ii, can be about 10%and even smaller, particularly, if the isolator is linearly polarized.As a result, the data regarding the power of input light signal Ii and,therefore, the data regarding the functionality of system 20 issubstantially more reliable than in the known prior art of powerfullaser systems.

Alternatively, either face 48 or face 50 of slitter 46 can be coatedwith a film. The coated face is thus operative to tap both the forwardpropagating and backreflected signals. Note that either coated oruncoated can tap the light. The faces 48 and 50 can extend in parallelplanes, as shown in FIG. 4. However, faces 48 and 50 can be configuredto extend in non-parallel planes.

FIG. 5 illustrates photodetector 44, which is configured, for example,as a pin photodiode. The percentage of tapped light Iti can be as smallas about half a percent of light signal Ii. However, even such anegligible portion of the Ii signal in powerful laser systems may bedetrimental to sensitive photo detectors. To avoid the possibility ofdestruction of photodiode 44, its surface may have a diffuser 54 formedby applying and cooling a drop of epoxy resin or any other materialcapable of adequately scattering the incident light. Experimental datashows that diffuser 54 provides for about 3-15 dB attenuation of thereflected signal while backreflecting a negligible portion of the Itisignal. The configuration of photo detectors 44 and 52 is identical and,in addition to being configured as a photodiode, can include any otherknown photodetecting element which may be provided with diffuser 54. Themeasurement of oppositely propagating forward and backreflected lightsignals may be simultaneous or sequential. While two detectors 44 and 52are shown, only one can be used for measuring the power of light signalpropagating in the desired direction, as known to one of ordinary skillsin the art.

Returning to FIG. 4, splitter 46 is preferably a relatively thick plate.Accordingly, photo-detectors 44 and 52, respectively, are axially offsetrelative to one another to accommodate for the thickness of splitter 46.

The above description of the power monitor unit including splitter 46and photodetectors 44, 52 relates to an optical isolator. However, asreadily understood by one of ordinary kills in the laser art, thedisclosed power monitor system may be easily associated with otheroptical elements, as discussed immediately below.

FIG. 6A diagrammatically illustrates an optical system including in partinput and output collimators 56, 58, respectively flanking an opticalfilter 64 and the disclosed power monitor unit which includes a splitter60 and one or two photodetectors 62. The splitter 60 is configured inaccordance with the above disclosed splitter. The optical filter 64 iswell known to one of ordinary skills in the laser art and needs not tobe disclosed in detail. FIG. 6B diagrammatically illustrates a furtherapplication of the disclosed power monitor unit including splitter 60and photodector(s) 62, which are located between input and outputcollimators 56, 58, respectively. In this configuration, the disclosedpower monitor unit functions simply as a power meter. FIG. 6Cdiagrammatically illustrates an optical circulator 66 located betweeninput and output collimators 56, 58, respectively, and optically coupledto the disclosed power monitor unit. As readily understood by one ofordinary skills in the laser art, other applications of the disclosedpower monitor unit can be easily envisioned.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the presently disclosedlaser powerful system. Thus, it is intended that the present disclosurecover the modifications and variations of this disclosure provided theycome within the scope of the appended claims and their equivalents.

1. A power monitor unit for a pigtailed optical system, comprising: aninput and output collimators spaced apart along an optical path; aplate-shaped beam splitter located between the input and outputcollimators and operative to tap off a portion of at least one offorward propagating and backreflected light signals; and at least onephotodetector optically coupled to the plate-shaped beam splitter andoperative to measure a power of the one optical signal upon receivingthe tapped-off portion of the light signal.
 2. The unit of claim 1,wherein the plate-shaped beam splitter has opposite faces lying inrespective planes, the respective planes extending transversely to aplane of propagation of the one light signal at an angle differing froma right angle.
 3. The unit of claim 2, wherein the planes of therespective opposite faces of the plate-shaped beam splitter are parallelto one another.
 4. The unit claim 2, wherein the planes of therespective opposite faces of the plate-shaped beam splitter arenonparallel to one another.
 5. The unit of claim 2 further comprising areflective film coated upon at least one of the opposite faces.
 6. Theunit of claim 2 further comprising a plurality of reflective filmscoated upon respective opposite faces of the beam-splitter.
 7. The unitof claim 1, wherein the one photodetector is a photo-diode provided witha diffuser, the diffuser being configured to attenuate the tapped-offportions of the respective forward propagating and backreflected lightsignals.
 8. The unit of claim 7 further comprising a second photo-diodeconfigured identically to the one photodetector, the one and secondphotodetectors being juxtaposed with the respective opposite faces ofthe plate-shaped beam splitter.
 9. The unit of claim 1, wherein the beamsplitter is directly optically coupled to the input and outputcollimators.
 10. The unit claim 1 further comprising an opticalcomponent located along the path light between the input and outputcollimators and optically coupled to the plate-shaped beam splitter, theoptical component being selected from the group consisting of an opticalisolator and a circulator.
 11. A pigtailed optical isolator for apowerful laser system comprising: an isolator core traversed by aforward propagating light signal along a light path; and a power monitorlocated along a downstream stretch of the core and including: abeam-splitter configured to simultaneously tap off portions of therespective forward propagating light signal and a backreflected lightsignal, and a photo-detector unit optically coupled to the splitter andoperative to provide information of powers of the respectiveforward-propagating and backreflected light signals.
 12. The opticalisolator of claim 11, wherein the beam-splitter has two opposite faces,at least one of the opposite faces having a reflective film coated uponthe one face, the opposite faces extending transversely to the lightpath at an angle differing from a right angle.
 13. The optical isolatorof claim 11 further comprising upstream and downstream collimatorsflanking the core, the core including at least one or multiple upstreampolarizes and a downstream polarizer spaced from one another and havingrespective polarizing coatings thereon, an optically active rotatorbetween the upstream and downstream polarizers and a Faraday rotatorbetween the optically active rotator and the downstream polarizer, thepower monitor being located between the downstream polarizer and thedownstream collimator.
 14. The optical isolator of claim 11, wherein thephoto-detector unit includes one or multiple photo-diodes each providedwith a diffuser, the diffuser being configured to attenuate thetapped-off portions of the respective forward propagating and reflectedsignals.
 15. The optical isolator of claim 14, wherein thephoto-detectors are axially offset along a longitudinal axis of theisolator core to accommodate for a thickness of the beam-splitter.
 16. Apowerful fiber laser system comprising: an optical component locatedalong a light path; and a plate-shaped beam splitter operative to tapoff a portion of at least one of a forward propagating and backreflectedlight signals; and a power monitoring unit operative to receive thetapped off portion so as to monitor a power of the one light signal. 17.The system of claim 16, wherein the plate-shaped beam splitter hasopposite faces lying in respective planes, at least one of the oppositefaces being coated with a reflective film, the respective planesextending transversely to a plane of propagation of the one light signalat an angle differing from a right angle.
 18. The unit of claim 17,wherein the planes of the respective opposite faces of the plate-shapedbeam splitter are parallel to one another or nonparallel to one another.19. The unit of claim 16, wherein the power monitoring unit isconfigured with one or multiple photo-diodes, at least one photo-diodebeing provided with a diffuser configured to attenuate the tapped-offportion of the one light signals
 20. The unit of claim 16, wherein theoptical component the optical component is selected from the groupconsisting of a linear-polarized optical isolator and a circulator. 21.A method of monitoring a power of light in a powerful fiber laser unit,comprising: tapping off a portion of at least one of forward-propagatingand backreflected light signals by a plate-shaped beam splitter locatedbetween input and output collimators; receiving the tapped off portionof the one light signal by a power measuring unit, thereby monitoring apower of the one light signal.
 22. The method of claim 21 furthercomprising tapping off portions of the respective forward propagatingand backreflected light signals and receiving the tapped off portion bythe power monitoring unit.
 23. The method of claim 22 further comprisingprocessing the received tapped off portions simultaneously orsequentially.
 24. The method of claim 21, wherein the plate-shaped beamsplitter has opposite faces extending in respective parallel ornonparallel planes.
 25. The method of claims 21 further comprising:locating the plate-shaped beam splitter between input and outputcollimators spaced apart along the path, and directly or indirectlyoptically coupling the plate-shaped beam splitter to the input andoutput collimators.